Three Rivers Cockle Abundance Survey Report April 2016

Transcription

1 Three Rivers Cockle Abundance Survey Report April 16 May 16 Crown Copyright WG 2884 ISBN digital

2 1. Introduction The annual Three Rivers Estuaries cockle abundance survey was conducted to assess cockle distribution, location, population size and estimation of tonnages, between April 18 th April 25 th 16. The survey is the latest in an 11 year series using an established and peer reviewed and tested standardised methodology. 2. Methodology The survey was carried out by Welsh Government Scientific and Enforcement Officers with assistance from WG Fisheries policy colleagues. The cockle beds were accessed at low water by 4 x 4 vehicles and were surveyed by foot. Welsh Government Fisheries Patrol RIB vessel support was utilised to safely access a large sand bank that has formed between Llansteffan and Ferryside (Middle Bank). An established m 2 grid covering the known cockle holding areas was used to act as a set of reference points and to ensure that the estuaries were adequately covered. The grid points were uploaded to a Garmin GPS Map 76c handheld GPS device for accurate location in the field. At each field station sediment from a.1m 2 quadrate was removed down to a depth of 5mm and was passed through a 5mm Endecott Scientific Sieve. The number and age of cockles, together with details of the sediment composition were recorded at each station, and representative samples were retained for determining population structure. The cockles retained were measured using Mytoya digital Vernier gauge for length frequency analysis. Following the m 2 survey at Laugharne and the Middle Bank, cockle retaining areas were additionally surveyed at a 5m 2 grid. The methodology was adapted in the field to ensure that dense cockle holding areas were accurately reflected in the analysis to improve confidence intervals. Cockle tonnages were then calculated using a length weight regression for Three Rivers Cockles (Hartley, 14), based on: [Mean cockle length] (Tonnage =mean wt (g) x area (m 2 ) x mean no. (no m 2 ) /1 x 6 )

3 3. Results Summary Statistics for all areas may be found in Table 1. Site Number per M 2 (Range) Mean Number per M 2 Modal Length (mm) Mean Length (mm) Llansteffan (1) Llansteffan (2) Gwendraeth Laugharne Ferryside St Ishmaels Llan y Bri Middle Bank Table 1. Summary Statistics for all areas (April 16) 3.1 Ferryside There were 4 stations in Ferryside that were holding cockle. Sediment was predominately muddy sand. There was a large range of the mean number per quadrate due to one station holding a larger number of Year 1 cockles (28). To mitigate the reduction of confidence in the data the mean number per station was determined by calculating the mean of all year classes and adding these calculations together. This gave a mean no per quadrate overall as 17, which was more representative of the cockle holding area Figure 1. Percentage Length Frequency Distribution of Cockles at Ferryside (n = 136) There were 2 year classes observed at Ferryside, and the bed was dominated by cockles of a spring 15 settlement, with the modal length of the cockle being 14mm (Figure 1). The smaller cockle ranges (-13) were most likely to be from a small Autumnal spat settlement.

4 There was no indication of an early spring 16 cockle spawning. The dominance of the larger cockle may indicate that survivability of cockle may have increased compared to in recent years. There were no incidences of atypical mortality observed. Mean tonnages at Ferryside were calculated as tonnes (Mean + SE). 3.2 St Ishmaels There were 6 stations at St Ishmaels that were holding cockle, with what is likely to be 2 length cohorts present (Figure 2) most likely comprising Spring 14 and Spring 15 cockle settlements. There was no evidence of an Autumnal spat settlement. Sediment ranged from coarse sand to sandy mud Figure 2. Percentage Length Frequency Distribution of Cockles at St Ishmaels (n = 83). Mean tonnages at St Ishmaels were calculated as 144 tonnes +.5 (Mean + SE). The distribution of the cockles at St Ishmaels linked up to the neighbouring cockle holding stations at Ferryside, with a similar length distribution pattern, albeit that smaller cockle were retained at the St Ishmael area. 3.3 Llansteffan There were7 stations in Llansteffan that were holding cockle. The sediment profile had a distinct bearing on the cockle densities with one station comprised of thick mud adjacent to the pill cutting through to the main channel harbouring the highest densities of cockle (84 per m 2 ). The large aggregation of cockles at this area may be preferentially associated with immersion time, and water flow velocity, both factors affecting rates of survivability and growth. Additionally the water flow velocity may be washing more cockles to within this area. Due to this station being significantly higher in density than other cockle holding stations at Llansteffan the station was analysed separately so as to now skew results for the remaining stations based on mean figures (Llansteffan 2)

5 Remaining stations had a mean number per m 2 of 4, resulting in an overall mean tonnage of tonnes at Llansteffan (Mean + SE) (Llansteffan 1) The length distribution profile of cockles at Llansteffan can be seen in Figure Figure 3. Percentage Length Frequency Distribution of Cockles at Llansteffan (n = 114). The length distribution at Llansteffan is dominated by Spring 15 cockle settlement. The smaller size classes may be a small Autumnal spat settlement. 3.4 Laugharne There had been a significant change in the topography of Laugharne since the last visit, with significant sediment loss resulting in the main channel cutting through the previously mapped sample stations. The sample stations that were accessible were not holding any cockle, however officers walked the full area and a small patch of cockle was identified. The perimeter of this area was walked (where possible, due to the sediment) and was estimated at 25 m x 6 m. 12 samples were taken within this small area. The sediment was heavy clay/mud with the cockles inhabiting it being dense and likely of a spring 15 settlement. They were stunted in growth, due to the anoxic nature of the sediment and the viscosity of the sediment inhibiting movement. It is likely that the cockles were experiencing limited food due to the tidal range, and also the sediment making it difficult for the cockles to be moved by tidal flow. Figure 4 shows the length distribution at Laugharne. The sediment can be viewed in Figure 5.

6 Figure 4. Percentage Length Frequency Distribution of Cockles at Laugharne (n = 332). Tonnages were calculated at Laugharne as (Mean + SE). Figure 5. Thick clay/mud sediment at Laugharne April Gwendraeth There were 3 stations at the Gwendraeth that held cockle. The numbers of cockles per M 2 (57-214) were reasonably dense, however the size of the bed does not harbour quantities sufficient for a commercial fishery.

7 Figure 6. Percentage Length Frequency Distribution of Cockles at Gwendraeth (n = 169). The size profile suggests that the Gwendraeth is dominated by Spring 15 cockle, potentially with a slower growth rate due to the position high up within the estuary. This would lead to reduced immersion time and subsequently reduced feeding. Tonnages were calculated at (Mean + SE). 3.6 Llan y Bri The cockle bed at Llan y Bri was split between 5 stations. The beds were of reasonable density, however again quantities are not sufficient to support a commercial fishery. The size profile (Figure 7) is subject to opinion Figure 7. Percentage Length Frequency Distribution of Cockles at Llan y Bri (n = 185).

8 There appears to be three length cohorts, comprised of a good quantity of larger cockle that may be a 14 settlement, with a range of cockle within the spring 15 timeframe. The smaller cockle may be from a limited autumn 15 settlement. Tonnages were calculated at (Mean + SE). 3.7 Middle Bank The middle bank is located between Ferryside and Llansteffan and is estimated to be approximately x 3 m (Figure 8) It is a large bank which is predominately sand sandy mud, with anoxic mud on the fringes of the bank. The bank was dispersed with cockle and there was no definitive dense cockle area. Nine Samples were then taken at random over the bank. Llansteffan Middle Bank Figure 8. Middle Bank between Llansteffan and Llan y Bri Tonnages on the Middle Bank were calculated at tonnes +.6 (Mean + SE). 3.8 Tonnage Calculations The combined tonnages for the full April 16 survey are shown in Table 2. Site Mean Tonnage Ferryside St Ishmaels 144 Gwendreath 3.5 Llan y Bri 462 Laugharne Llansteffan Middle Bank Total Tonnage Table 2 Tonnages of all sites

9 Based on the tonnage indications from the current survey, there is not sufficient cockle to warrant opening a commercial fishery. To facilitate management a growth prediction of 4mm was added to the mean length of each of the cockle beds. This was used to give an indication of estimated tonnages available by August 16. This is a very arbitrary prediction and is very subjective. It utilised growth profiles from previous years, but does not take into account different growth rates between the length cohorts. Based on 75% survivability (subject to natural mortality and predation) an estimated tonnage of 2498 tonnes would be the total of all sites in August. This predicted tonnage is still not sufficient for commercial gathering. 3.9 Discussion The Three Rivers Estuaries are an important overwintering site for Oystercatchers. The advice from Natural Resources Wales, Welsh Government s Statutory Nature Conservation Advisors, is that a minimum of 369 tonnes of cockle, above 15mm, is required for food to sustain these birds through the winter. This is currently the best available evidence and WG are utilising this advice in conjunction with a pan Wales approach to bird food and cockle gathering. With this in mind it is doubtful that a fishery will be able to occur in 16. The last cockle fishery took place in the Three Rivers in June 12. This fishery opened on the basis of there being ca. 7 tonnes of stock, of which 26 was of fishable size (>21mm). This is significantly greater than the tonnage available in the current survey and within the growth prediction for August 16. Nevertheless the cockle beds will continue to be monitored for growth and mortality on a monthly basis. Any changes will be documented and if necessary an additional full survey will be conducted in August 16. It is recommend on a scientific basis that the cockle beds remain closed with management decisions reviewed through the forthcoming summer months. Dr Leanne Llewellyn May 9 th 16